No, freckles cannot be cancerous. The important thing is to see a dermatologist, so the dermatologist can distinguish freckles from brown spots that may be pre-cancerous or cancerous.
Visit a Dermatologist
What I tell patients to do is before embarking on freckle treatment is to simply visit their dermatologist so that they can give their skin a look and see if there is anything that needs to be sampled. We need to do this before the skin is treated because once the skin is treated by a laser, spots will not look the same.
Hard to Decipher Freckles from other Spots
The problem is that non-dermatologists, or even dermatologists, often have a hard time distinguishing brown spots that need closer attention from brown spots that do not. So see your dermatologist before embarking on treatment of any brown spots, including freckles.